The present invention relates to an apparatus for on-line measurement of flow rates of respective fluids of a multiphase fluid comprising oil, water, gas, etc. and flowing in a pipe, without separating the respective fluids.
Conventionally, a flow meter for a multiphase flow composed of three or more sensors such as a water cut meter utilizing a difference in electrical properties among fluids, a density meter utilizing differences in density among fluids, and a flow meter measuring a total flow rate or a flow velocity of a multiphase fluid has been employed for measuring flow rates of respective fluids constituting a multiphase fluid.
FIG. 7 is a block diagram of such a conventional flow meter for a multiphase flow. This flow meter for a multiphase flow is constituted by a total of three sensors, that is, a cross-correlation flow meter 6 composed of two capacitance water cut meters 3 and 3xe2x80x2 and a gamma-ray densitometer 7 for measuring an average density of a multiphase fluid 2.
These capacitance water cut meters 3 and 3xe2x80x2 are composed of electrodes 4 and 4xe2x80x2 and impedance measurement circuits 5 and 5xe2x80x2 provided in a pipe 1, and the gamma-ray densitometer 7 is composed of a source of gamma rays 8 and a detector 9.
An absolute pressure meter 10 and a thermometer 11 are used for temperature correction of parameters such as density and dielectric constant of respective fluids and a (volume) flow rate of gas.
Now, a principle of measurement for the conventional apparatus is described.
An electrostatic capacity C of a multiphase fluid 2, which consists of oil, water, and gas and flows in a pipe 1, is measured with a capacitance water cut meter 3 and a transmittance xcex for gamma rays of the multiphase fluid 2 is measured with a gamma-ray densitometer 7. Equation (1)
HP+HW+HG=1xe2x80x83xe2x80x83(1):
wherein HP represents an oil phase ratio, HW represents a water phase ratio, and HG represents a gas phase ratio for the multiphase fluid 2, is established.
When known relative dielectric constants of oil, water, and gas are expressed by xcex5P, xcex5W, and xcex5G, Equation (2):
xcex5PHP+xcex5WHW+xcex5GHG=fxcex5(C)xe2x80x83xe2x80x83(2)
is established for the relationship between the known relative dielectric constants and the electrostatic capacity C measured.
When known densities of oil, water, and gas are expressed by xcfx81P, xcfx81W, and xcfx81G, Equation 3:
xcfx81PHP+xcfx81WHW+xcfx81GHG=fxcfx81(xcex)xe2x80x83xe2x80x83(3)
is established for the relationship between the known densities and the gamma-ray transmittance xcex measured.
Then, fxcex5(C) and fxcfx81(xcex) are intrinsic functions of the capacitance water cut meter 3 and the gamma-ray densitometer 7, and provide an average dielectric constant of the multiphase fluid 2 from an electrostatic capacity C and an average density of the multiphase fluid 2 from the transmittance xcex, respectively.
On the other hand, a cross-correlation flow meter 6 composed of two capacitance water cut meters 3 and 3xe2x80x2 measures a travel speed of fluctuations of electrostatic capacity C, that is, an average flow velocity u of the multiphase fluid 2.
An arithmetic circuit 12 to calculate flow rates of the respective phases calculates an oil phase ratio HP, a water phase ratio HW, and a gas phase ratio HG for the multiphase fluid 2 from the simultaneous equations (1)-(3) and then calculates flow rates of the respective fluids QP, QW, and QG from Equations (4-1), (4-2), and (4-3) using a cross section A of the pipe 1 and the average flow velocity u.
QP=HPxc2x7Axc2x7uxe2x80x83xe2x80x83(4-1)
QW=HWxc2x7Axc2x7uxe2x80x83xe2x80x83(4-2)
QG=HGxc2x7Axc2x7uxe2x80x83xe2x80x83(4-3)
A method of obtaining a flow velocity from fluctuations of a multiphase fluid is described in detail in Japanese Patent Application No. 8-128389, etc.
However, such a conventional apparatus requires a combination of at least three sensors such as two capacitance water cut meters and a gamma-ray densitometer to obtain component ratios and average flow velocities of respective fluids constituting a multiphase fluid and thus interferes with simplification and size-reduction of a structure of a flow meter for a multiphase flow.
In addition, since an average flow velocity u is solely used to calculate flow rates, there has been such a problem that errors in flow rates of respective fluids measured become larger when a velocity slip (difference in velocity) exists between gas and liquid.
It is an object of the present invention to provide a flow meter for a multiphase flow composed of a smaller number of sensors than the conventional apparatuses and to provide a flow meter for a multiphase flow enabling highly accurate measurement in spite of velocity slip between gas and liquid.
According to the method of the present invention, a cross-correlation flow meter for measuring basic values to calculate component ratios of respective fluids constituting a multiphase fluid comprising a gas and a plurality of liquids is provided to obtain the component ratios of the respective fluids on the basis of the measured values of the cross-correlation flow meter; when there is no velocity slip between a gas phase and a liquid phase in the multiphase fluid, an average flow velocity of the multiphase fluid is obtained on the basis of time between fluctuations of the measured values of the cross-correlation flow meter and then flow rates of the respective fluids are obtained by utilizing the respective component ratios and the average flow velocity; and when there is a velocity slip between the gas and liquid phases, a flow velocity of the gas phase of the multiphase fluid is obtained on the basis of time between fluctuations of the measured values of the cross-correlation flow meter, and a sensor for measuring basic values to calculate a flow velocity of the liquid phase of the multiphase fluid is provided to obtain the flow velocity of the liquid phase on the basis of the measured values of the sensor, and then the flow rates of the respective fluids are calculated by utilizing the component ratio of a fluid in gas phase, the flow velocity of the gas phase, the component ratios of fluids in the liquid phase and the flow velocity of the liquid phase.
The cross-correlation flow meter comprises two component ratio meters for measuring predetermined electrical values in a pipe through which the multiphase fluid flows, and the component ratios of the respective fluids are obtained by acquiring information concerning component ratios of the fluids in the liquid phase components from both measured values obtained by the component ratio meters at an instance when the pipe is filled with liquid alone during the passage of the multiphase fluid through the pipe and electrical characteristic values of the respective fluids, acquiring information concerning the component ratios of the respective fluids from both a time average of the measured values obtained by the component ratio meters and the respective electrical characteristic values, and utilizing the fact that a sum of the component ratios of the respective fluids becomes 1.
In addition, the cross-correlation flow meter comprises two radiation densitometers for measuring radiation transmittance in a pipe through which the multiphase fluid flows, and the component ratios of the respective fluids are obtained by acquiring information concerning component ratios of the fluids in the liquid phase from both measured values obtained by the radiation densitometers at an instance when the pipe is filled with liquid alone during the passage of the multiphase fluid through the pipe and densities of the respective fluids, acquiring information concerning component ratios of the respective fluids from both a time average of the measured values obtained by the radiation densitometers and the respective densities, and utilizing the fact that a sum of the component ratios of the respective fluids becomes 1.
A differential pressure of the multiphase fluid is measured with a differential pressure type flow meter and a flow velocity of the liquid phase is obtained on the basis of the measured differential pressure, an average density of the multiphase fluid, and an intrinsic coefficient for the differential pressure type flow meter.
The apparatus of the present invention comprises a cross-correlation flow meter provided in a pipe through which a multiphase fluids comprising a gas and plurality of liquids, for measuring basic values to calculate component ratios of respective fluids constituting the multiphase fluid; and an arithmetic circuit for calculating flow rates of the respective fluids, by acquiring information concerning component ratios of fluids in a liquid phase of the multiphase fluid from both measured values obtained by the cross-correlation flow meter at an instance when the pipe is filled with liquid alone during the passage of the multiphase fluid through the pipe provided with the cross-correlation flow meter and characteristic values of the respective fluids of the multiphase fluid, acquiring information concerning the component ratios of the respective fluids from both a time average of the measured values obtained by the cross-correlation flow meter and the respective characteristic values, obtaining the component ratios of the respective fluids by utilizing the fact that a sum of the component ratios of the respective fluids becomes 1, calculating an average flow velocity of the multiphase fluid on the basis of time between fluctuations of the measured values obtained by the cross-correlation flow meter, and utilizing the respective component ratios and the average flow velocity for the calculation of the flow rates.
The apparatus of the present invention comprises a sensor provided in a pipe, for measuring basic values to calculate a flow velocity of a liquid phase of a multiphase fluid; and the arithmetic circuit for calculating flow rates of the respective fluids with an additional function to calculate a flow velocity of a gas phase of the multiphase fluid on the basis of time between fluctuations of the measured values of the cross-correlation flow meter when there is a velocity slip between a gas phase and a liquid phase of the multiphase fluids, to calculate a flow velocity of the liquid phase on the basis of the measured values obtained by the sensor, and to calculate flow rates of the respective fluids by utilizing the component ratio and the flow velocity of the gas phase and the respective component and the flow velocity of the liquid phase.
In addition, the cross-correlation flow meter comprises two component ratio meters for measuring an electrostatic capacity of the multiphase fluid and the characteristic values are in relative dielectric constant. in addition, the cross-correlation flow meter comprises two radiation densitometers for measuring radiation transmittance of a multiphase fluid and characteristic values are in density.
Each of the component ratio meters comprises a cylindrical driving electrode for applying a voltage signal of predetermined amplitude and frequency to the multiphase fluid and a cylindrical measurement electrode virtually grounded for detecting a current flowing in through the multiphase fluid, both electrodes being arranged in parallel with the pipe through which the multiphase fluid flows, so as to measure a water phase ratio in the multiphase fluid by measuring an electrostatic capacity between the cylindrical driving electrode and the cylindrical measurement electrode, wherein a cylindrical dummy electrode with a potential identical to that of the cylindrical measurement electrode is placed between the cylindrical driving electrode and the cylindrical measurement electrode so as to reduce a part of an electric line of force toward the cylindrical measurement electrode through the vicinity of the wall of the pipe.
When an inner diameter of the pipe and inner diameters of the respective cylindrical electrodes are expressed by D, the widths of the cylindrical driving electrode, the cylindrical measurement electrode, and the cylindrical dummy electrode are expressed by ls, lm, and ld, respectively, a distance between the cylindrical driving electrode and the cylindrical measurement electrode is expressed by L, and a distance between the cylindrical driving electrode and the cylindrical dummy electrode is expressed by x,
ls/D=0.3-1.0
lm/D=0.3-1.0
ld/D=0.1-0.5
L/D=1.0-2.0
x/D=0.4-1.2.
The constitution of the present invention enables to constitute a flow meter for a multiphase flow with a smaller number of sensors than conventional ones and thus enables simplification and size-reduction of apparatuses. In addition, even when there is a velocity slip between gas and liquid, highly accurate measurement of flow rates of the respective phases can be achieved by only adding a sensor. Furthermore, the accuracy of measurement of flow rate is enhanced through improvement of a component fraction meter.